Table of contents

The measure of the local field probability density in the discrete (+or-h) short-ranged coupled random-field Ising chain is studied with the use of a specific discrete one-dimensional map. The strange set corresponding to this measure resembles Cantor sets and its full scaling properties are computed by evaluating the generalised fractal dimensions D_q and the spectrum of singularities f( alpha ). The main results are also derived by perturbation theory valid for weak fields h.

The authors show that a magnetic field parallel to a material interface generates a complex Landau level structure for a conduction electron near the interface. This is shown to result from the existence and interaction of current-carrying interface states with pseudo-bulk Landau states.

The authors examined the mechanism for, and the nature of pairing in the newly discovered layered oxide superconductors modelled by a two-dimensional Hubbard Hamiltonian having a low concentration n_h holes in the lower Hubbard band. They find spin-singlet d wave pairing. The calculated binding energies typically approximately=100 K show a maximum at n_h approximately=0.15 per atom, in accord with experiment. The pairing potential originates from a wake of higher energy spin configurations generated by a moving hole.

The authors report the effects of distortions of crystal lattices on the superconductivity of Y-Ba-Cu-O system compounds under different treatment conditions (cooling rates and aging atmosphere). The results show that with an increase of cooling rate the intensities of three dominant X-ray diffraction peaks in the orthorhombic perovskite phase gradually become inverted, which corresponds to the changes in the superconductivity. It has been found that there exists an intense interaction between Y-Ba-Cu-O compounds and polarisable molecules. The interaction may result in the distortion of crystal structure; therefore the superconductivity vanishes in the range above liquid nitrogen temperature. This suggests that there is a close relationship between the distortion of the crystal structure and the superconductivity of the compounds. Based on the experimental data, a quantitative calculation is obtained. The qualitative explanation is suggested based on polarisation theory and crystal-field theory.

Phase transitions of Y₁Ba₂Cu₃O_9-x compounds and influencing factors have been studied using an X-ray diffractometer equipped with a high-temperature stage. The results showed that there exist three zones of phase transition from room temperature to 1000 degrees C in the compounds. From 400 to 600 degrees C, the transition from orthorhombic to tetragonal phases occurs accompanied by an oxygen release process. Above 950 degrees C, the tetragonal phase is transformed into another high-temperature phase.

Some EPR and NMR experiments at the Cu sites in the mixed-phase materials Ba_0.6Y_0.4CuO_y and Ba_0.2Y_0.8CuO_y, and the pure-phase compounds YBa₂Cu₃O_y (1:2:3), BaCuO₂ (0:1:1), Y₂Cu₂O₅ (2:0:2), Y₂BaCuO₈ (2:1:1), YBa₃Cu₂O_y (1:3:2) and CuO are presented and discussed. In particular, it is shown that any EPR signal in nominally pure 1:2:3 superconducting material is probably due to a small amount of impurity 2:1:1 (greenphase) compound. For the 2:1:1 and 1:3:2 compounds the authors find g/sub /// approximately=2.23 and g_{perpendicular to} approximately=2.09 with a linewidth of approximately=0.025 T, whereas for the 2:0:2 compound g=2.12(2) with a linewidth of 0.047(3)T. Some suggestions are also made concerning the absence of both NMR and EPR signals in the 1:2:3 compound, in terms of (i) Anderson's resonating bond model and (ii) recent FLAPW band-structure calculations. In particular, it is argued that the delocalised nature of the Cu d electrons probably gives rise to exceptional spin-spin dipolar broadening of both the NMR and EPR transitions.

A superconducting transition with an onset temperature higher than 80 K has been found in Y-Ba-Cu-O systems. Y_2.4Ba_3.6Cu₆O_{14- delta} shows a weak Meissner effect whereas in YBa₂Cu₃O_{8- delta} the Meissner signal is 10% of the complete diamagnetism. For the latter system the critical fields have been determined from magnetisation measurements and are reported here.

In the superconducting state, YBa₂Cu₃O₇ absorbs electromagnetic radiation over a wide range of frequencies (8 MHz-9 GHz). The absorption is extremely sensitive to temperature, particle size and the magnetic field and depends crucially on the presence of oxygen. A possible explanation for the phenomenon based on the formation of Josephson junctions is suggested.

Optical second harmonic generation was measured across the ferroelectric phase transition in PbHPO₄ under an applied electric field. The authors show that linear coupling of nonlinear susceptibility to the order parameter describes well the temperature and electric field dependence of d₁₁₁. This indicates simultaneous reorientation of heavy ions and ordering of protons, contrary to recent Raman and neutron scattering results. Some residual SHG generation can be obtained in the para-electric phase with no applied electric field in newly polished and mounted samples. This SHG signal is attributed to the perturbation of the crystal by defects, which can be annealed nearly completely with a pulsed electric field. P2/c macroscopic symmetry is deduced for the para-electric phase.

A simple correlated valence fluctuation model is proposed as a possible mechanism for high-T_c superconductivity in certain copper oxides. Electron pairs are formed by Coulomb attraction between electrons in mainly Cu²⁺ (d⁹ configuration) pyramidal planes mediated by Cu³⁺ (d⁸ configuration) ions in the CuO₄ chain-like plane in YBa₂Cu₃O_7-x. These electron pairs have high binding energies necessary for high T_c, and the model is capable of explaining a number of other observed properties.

A deformation dipole model for the lattice dynamics of KCN in an orientationally disordered phase has been formulated. The model includes the rotational-translational coupling effect based on the theory of Michel and Naudts (1977). The authors report phonon dispersion curves of KCN at 300 K and compare them with the available experimental data. The agreement is very good. The one-phonon density of states of KCN is also reported.

X-ray scattering measurements have been performed on the perovskite KMnF₃ to investigate the critical behaviour at the cubic-to-tetragonal structural phase transition at 186 K. The critical exponents describing the temperature dependence of the order parameter, the correlation length and the susceptibility have been determined. An anomalous second component was observed in the scattering cross section close to the transition temperature, in addition to the anisotropic diffuse scattering from the softening R₂₅ normal mode. The anomalous component was well described by an isotropic Lorentzian-squared lineshape.

The isomorphous transformation of CeP previously reported near 10 GPa is confirmed but the relative volume decrease Delta V/V₀ is reduced from 7.5% to 3% because of a smaller bulk modulus at lower pressures. This difference is attributed to large anisotropic stress components which were not taken into account in previous experiments. The cubic NaCl-type phase is stable up to 19 GPa at which a crystallographic change to the cubic CsCl-type structure occurs with a volume decrease of 10.5%. The results are discussed in terms of the valence change and the Kondo concentrated-system models.

The valence band structures of A^IB^IIIC₂^VI (A^I=Cu, Ag) type compounds are determined with electron state hybridisation phenomena, in particular with d-p resonance hybridisation phenomenon. In the region of the noble metal atom d band localisation the influence of this phenomenon on the density of valence band states curve in Cu compounds is greater than in Ag compounds; this is explained by the lowering of binding energy of the Cu d state centre of gravity in comparison with the Ag d state in the compounds investigated. The hybridised d-p resonance gives rise to p state splitting of the anion and of the other atoms of the compound, and (together with the influence of the crystal field) determines the energy splitting of the noble metal d states in the compounds investigated.

The one-dimensional electron system with both backward- and forward-scattering impurities is considered, using the Fukuyama-Lee method for a weakly localised system, in which the impurity density is large and the potential is weak. When the fluctuation of impurity number in the characteristic length L₀(n_iL₀>>1, n_i being an impurity concentration) can be neglected, it is demonstrated that large solitons whose size is of the same order as L₀ appear because of the forward-scattering by impurities, and consequently the system becomes metallic. A combined effect of an incommensurate lattice and impurities is also considered. When the impurity potential is much stronger than the lattice potential, the system is always localised, while in the converse case the system becomes metallic by forming solitons.

Optical phonon spectra are investigated by infrared reflectivity measurements for La_1-xSr_xFeO₃ (x=0, 0.1, 0.25 and 0.4) and La_1-xSr_xCoO₃ (x=0, 0.02 and 0.1). Using the phonon parameters determined by Kramers-Kronig and oscillator analysis, the effective charges and the electron-phonon coupling constants are evaluated for each phonon mode. On the basis of these results, the chemical bonding and the electronic band structure of these materials are discussed. It is found that the bonding in LaCoO₃ is covalent and that it is not affected by the Sr substitution. However, the bonding in LaFeO₃ is rather ionic and the Sr substitution increases the degree of the covalency in this material.

On the basis of the considerations of Tutto and Zawadowski (1985), the temperature-dependent threshold electrical field for a charge-density wave is studied both for strong and for weak single impurities. Below the transition temperature a slight decrease in the threshold field with increase in the temperature is found, and this disagrees with experimental results.

MBM(TCNQ)₂ is a charge transfer salt with two inequivalent types of uniform TCNQ stacks. The authors report measurements of the current noise power spectra of this compound. Two lorentzian contributions to the noise spectrum could be observed. They argue that the noise measurements should be interpreted as shot noise. In this picture, the lifetimes of the charge carriers can be longer than the transit time due to reflection at the contact boundaries. The measurements indicate that the charges are fractional (q=+or-¹/₂e). An explanation is proposed in terms of electric transport by solitons with fractional charges and extremely long lifetimes. Although the soliton picture is, at this stage, rather speculative, it appears to be the only charge-transport mechanism currently to be found in the literature that is compatible with the experimental results.

Shot noise was observed in the electric transport in uniform MEM(TCNQ)₂, MEtM(TCNQ)₂ and MBtM(TCNQ)₂. The shot noise occurred in the ohmic regime. The mean distances that the charge carriers travel between trapping events are larger than the lengths of the samples ( approximately 3 mm). An explanation in terms of solitons with charges +or-¹/₂e is proposed. Although the soliton picture is, at this stage, rather speculative, it appears to be the only charge-transport mechanism currently to be found in the literature that is compatible with the experimental results. In MEtM(TCNQ)₂ and MBtM(TCNQ)₂ two conduction mechanisms were found, but the precise nature of these mechanisms and their interactions could not be revealed.

This paper analyses the role of defects within the flux-line lattice of high-current-density superconductors. A simple model demonstrates that above the normal density distribution in critical currents the E-J characteristics is linear with a resistivity many orders of magnitude less than the bulk flux-flow resistivity. The universality of the three free parameters that describe the defect motion is explicitly demonstrated by considering data on Nb₃Sn throughout its entire superconducting phase.

Doping of ZnSe with gold results in the appearance of three broad emission bands which peak at 575 nm, 605 nm and 695 nm (at 2 K). In ODMR experiments each band gives rise both to shallow-donor and to deep-centre magnetic resonance transitions and each is thus identified as being due to donor-deep-centre recombination. The deep centre (Au-1) associated with the 605 nm band has orthorhombic symmetry with mod g_x mod =2.226, mod g_y mod =2.156 and mod g_z mod =1.950, where y is along a (110) direction and where x and z are tilted by 8 degrees from (112) and (111) directions respectively. For the 695 nm band the deep centre (Au-2) is unusual in having axial symmetry about a (100) axis, with mod g/sub /// mod =2.183, mod g_{perpendicular to} mod =2.033, mod A/sub ///(¹⁹⁷Au) mod , <10*10^-4 cm^-1 and mod A_{perpendicular to} (¹⁹⁷Au) mod =24*10^-4 cm^-1. For the 575 nm band the deep centre (Au-3) is again orthorhombic, with mod g_x mod =2.002, mod g_y(100) mod =2.010 and mod g_z mod =2.083, with the x and z axes tilted from (112) and (111) directions by 20 degrees . The Au-1 centre is ascribed to a gold-containing A centre, the Au-2 to interstitial gold in octahedral selenium coordination and the Au-3 to substitutional gold associated with a perturbing defect.

The authors report on linear birefringence (LB) measurements in the dilute archetypal 2D Heisenberg ferromagnet K₂Cu_xZn_1-xF₄ in the whole concentration range 0<x<1. From the temperature dependence of the LB in the range 1.3-300 K, which is separated into two parts (the lattice contribution and the magnetic contribution), they discover information on the influence of the Jahn-Teller effect and on the change in the magnetic dimensionality with dilution. Moreover, they confirm previous specific heat results at low temperatures. Since the magnetic LB is proportional to the magnetic energy, they determine and explain its dependence on dilution.

The authors studied the influence of the gradual decrease in electronic density of the surface, approximated by a two-step model, on the dispersion relation of the dipole modes of a metallic sphere.

Experiments, using the frequency-crossing technique, on phonon thermalisation in an Al₂O₃ bicrystal indicate that at 2 K and frequencies of around 200 GHz the process occurs predominantly at the surfaces. The high probability (about 1) of inelastic scattering at a fine-ground cylindrical surface is not reduced by annealing in argon at 1775 degrees C nor by a Syton polish followed by a hydrogen anneal at 1200 degrees C. Preliminary information is obtained on the spatial growth of holes in a phonon current as it travels through a resonantly scattering medium.